The invention relates to a dryer with a drying chamber for the items to be dried and a process air duct, in which are located a heater for the heating of process air and a blower for forcing the process air from the heater through the drying chamber and a heat exchanger arrangement, by means of which heat exchanger arrangement heat can be removed from the process air flowing from the drying chamber, and fed to the process air flowing to the heater, where the process air duct has an inlet air opening for drawing in process air from an environment of the dryer and an exhaust opening for expelling process air into the environment of the dryer.
The invention also relates to a method for operating such a dryer.
Such a dryer and such a method follow from DE 30 00 865 A1.
In general a tumble dryer is operated as an exhaust air dryer or a condensation dryer. An exhaust air dryer directs heated air once through the laundry to be dried and conveys this moisture-laden air through an exhaust air hose from the exhaust air dryer and out of the room in which it is set up. A condensation dryer, whose method of functioning relies on the condensing of the moisture evaporated out of the laundry by means of the warm process air, requires no exhaust air hose and enables the recovery of energy from the heated process air, for example through the use of a heat pump. Generally, though, it is necessary in such condensation dryers to collect the condensate accruing, and either pump it out or dispose of it through the manual emptying of collector tanks.
DE 40 23 000 C2 describes a tumble dryer with a heat pump circuit in which an inlet air opening is arranged in the process air duct between the condenser and the evaporator, which can be closed by means of a controllable sealing device.
In an exhaust air dryer, on the other hand, the air which is moisture-laden after passing through a laundry drum is generally directed out of the dryer. Compared with a condensation dryer, an exhaust air dryer can be more simply and thus more cheaply constructed. During its operation, an exhaust air dryer draws air from its environment, and uses this directly for drying purposes. As a rule, such ambient air is comparatively dry, with a relative humidity of significantly less than 100%, and can thus absorb a relatively large amount of moisture. By comparison, the circulating process air in a condensation dryer can as a rule only be dehumidified at 100% relative humidity at the lowest temperature obtaining in the process air circuit, which limits its capacity for absorbing moisture from the items to be dried and thus restricts the drying process to a certain extent.
Heat recovery is also possible in principle in an exhaust air dryer, though each such heat recovery implies a cooling of the exhaust air, whereby condensate can precipitate out of the exhaust air and has to be disposed of.
DE 30 00 865 A1 discloses a tumble dryer with heat recovery. The tumble dryer comprises a container to accommodate and move the laundry, into which is fed a heated supply air flow from a heating element, while the moist and warm air is conveyed as exhaust air via an outlet. A heat exchanger is arranged in the supply air flow upstream of the heating element, through which flows the humid exhaust air from the container. The tumble dryer is embodied as an exhaust air dryer.
In an exhaust air dryer with heat recovery, the ambient air (for example from 20° C. and 60% relative humidity; so-called supply air) as a rule flows into the heat exchanger surfaces of an air-to-air heat exchanger, where it is heated up accompanied by cooling of the warm process air emerging from the drying chamber. Depending on the cooling power or heat exchange respectively, condensate accrues, which is collected in a container or pumped out. In the first case emptying is necessary and in the second case connection to the wastewater network. The amount of condensate accruing is a gage of the heat energy given off in the heat exchanger and thus a measure of the improvement in energy efficiency.